The polyamide industry uses an entire range of monomers formed from diamines and from diacids, from lactams, and especially from α,ω-amino acids. These monomers are defined by the length of the methylene chain (—CH2)n separating two amide functions —CO—NH—.
For the purposes of the invention, the term “α,ω-aminoalkanoic acids or esters”, hereinafter simply “amino acids or amino esters”, is in fact intended to mean any long-chain α,ω-amino acid, that is to say the chain of which comprises at least 8 carbon atoms.
Indeed, the polyamides targeted by the present invention are technical polyamides, that is to say high-performance, or even very-high-performance, polyamides produced from monomers comprising at least 8 carbon atoms, preferably at least 10 carbon atoms; as opposed to the “commodity” polyamides, such as nylon 6, of which the amounts (volumes) sold are much greater and the costs much lower than those of the technical polyamides.
Polymerization from long-chain α,ω-aminoalkanoic acid or ester monomers, in order to achieve high degrees of polymerization (greater than 80), requires monomers of high purity.
Among the most promising processes for producing long-chain α,ω-aminoalkanoic acid or ester, mention may be made of processes for hydrogenation of unsaturated acid nitriles or unsaturated nitrile esters, such as those described in patent applications WO 08/104722 and WO 14/122412. These patent applications describe a process for synthesis of amino acid by metathesis, hydrolysis, then hydrogenation.
During tests for producing polyamides from these long-chain α,ω-aminoalkanoic acids or esters, two problems have been revealed. In some cases, it is not possible to obtain polyamides with a high degree of polymerization. In other cases, the melt rheology of the polyamide obtained is unstable at high temperature, about from 200 to 350° C. for example: the melt viscosity tends to increase. This instability can, during the conversion of the granules into molded or extruded pieces (and in particular during an unforeseen machine stop), lead to a blocking of the injection or extrusion screws because of the excessively high viscosity reached. This blocking has several drawbacks: It is necessary to disassemble and clean the machine before starting up again, which leads to a loss of production time, or even a degradation of the material.
There is therefore a real need to find starting materials based on long-chain α,ω-aminoalkanoic acid or ester which make it possible to produce long-chain polyamides with a high degree of polymerization, and which are stable at high temperature, that is to say the melt viscosity of which does not increase.
The applicant has now found a composition based on long-chain α,ω-aminoalkanoic acid or ester, which makes it possible to achieve these two objectives, by combining the α,ω-aminoalkanoic acid or ester with a particular content of alkanoic acid or ester. The latter has in fact been identified by the inventors as a key element of the composition for producing long-chain polyamides with a high degree of polymerization and which are stable at high temperature.